Geometric parameters of micro channel aspect ratios are determined for heat exchangers for gaseous fluids in which micro channels have a surface area density greater than 10000 m.sup.2/m.sup.3 in the alternate situations a) where volume is constant, and b) where volume is variable. Computational fluid dynamics and an analytical approach are combined under given constraints to optimize micro channel aspect ratio and micro channel spacing using plots of the performance curves of pressure loss in the channel for the hot side; pressure loss in the channel for the cold side; heat flux; and heat transfer rate.